1. Technical Field
The present disclosure relates to a semiconductor circuit, and more particularly, to a semiconductor circuit for reducing flicker noise or phase noise, and a voltage controlled oscillator (VCO) including the semiconductor circuit.
2. Discussion of the Related Art
Voltage controlled oscillators (VCOs) have output signals with desired oscillation frequencies in response to external control voltages, and are used, for example, in phase-locked loops (PLLs), analog sound mixing apparatuses, and mobile terminals.
The phase noise of a VCO is important as affecting the performance of the VCO. Accordingly, the less the phase noise of a VCO, the better the performance of the VCO. The main cause of the phase noise is called “flicker noise” or “1/f noise”.
FIG. 1 is a circuit diagram of a negative conductance generator implemented in a conventional VCO 10. Referring to FIG. 1, the VCO 10 includes a negative conductance generator 20 and an LC tank circuit 30.
The negative conductance generator 20, which is a frequency source, generates a signal with a predetermined frequency using negative conductance. The LC tank circuit 30 includes inductors L and capacitors C, and generates a signal with a desired frequency, in response to the signal with the predetermined frequency and a control voltage Vctl.
Referring to FIG. 1, since each body of cross-coupled NMOS transistors 21 and 23 is grounded, the NMOS transistors 21 and 23 are adversely affected by flicker noise. Accordingly, the VCO 10 including the cross-coupled NMOS transistors 21 and 23 is vulnerable to phase noise.